The invention relates to a process for resistance welding with a welding current which pulsates in periodic half-waves, in particular an alternating current, and which is generated from a primary alternating voltage and is controlled by pulse duration modulation thereof.
The invention furthermore relates to an arrangement for carrying out the process, having a static frequency changer, which has a direct current intermediate circuit and, as an output stage, has a chopper which generates the primary alternating voltage and transmits it to the welding transformer whose secondary circuit is connected to welding electrodes of a resistance welding machine.
Such a process and such an arrangement are known from EP-A2-0 260 963, which will be discussed in greater detail hereafter.
In one known seam-welding device (EP-A1-0 261 328) for the resistance-longitudinal seam welding of the overlapped edges of bodies for conserve cans and the like, a three-phase mains alternating voltage is converted into a direct current voltage, which is smoothed and converted into an impulse voltage with an alternating polarity. This impulse voltage is applied to the welding electrodes of the seam-welding device. The frequency of the impulse voltage is so selected, that the resulting welding current is continuous and for this reason the individual welding nuggets or spots, which in each case are created by one of the rectangular half-waves of the impulse voltage, overlap each other. As each welding current half-wave is generated by a half-wave of the impulse voltage, the welding current form is dependent upon the duration of the impulse voltage half-waves. If, when regulating the welding current, this impulse duration is varied during a half-wave, this leads to a considerable variation in the welding current form, which must be regarded as disadvantageous. It would be considerably more advantageous if the welding current form were not dependent on and therefore not determined by machine parameters, but could be preselected for optimizing the welding result.
Furthermore, in the known seam-welding device the welding current regulator, which controls the current per welding spot, operates in each case with the measured value of the preceding welding spot. The reaction time of the regulator, also determined by a correction device which is controlled by the latter, is therefore relatively long (with a welding frequency of 500 Hz, the reaction time reaches 1 ms). As a result, the regulator is not in a position to correct rapid variations of the welding parameters (e.g. for a contaminated sheet metal surface). In order to improve the regulating ability of this known seam-welding device, the regulator reaction time would therefore require to be shortened. To this end, the switching frequency could conceivably be increased by a certain factor. However, the frequency of the welding current would also be increased by this factor as a result. In consequence of the strongly inductive load of the seam-welding device, because the impedance would increase proportionally to the frequency, the welding current is reduced by a factor which would be equal to the reciprocal of the factor by which the switching frequency would have been increased. In order to compensate this, the voltage and the power of the frequency changer and the welding transformer of the known seam-welding device would have to be increased by the same factor by which the switching frequency was increased. Additionally, the requirement that the welding frequency should stand in a certain ratio to the welding speed, would no longer be fulfilled. In this known seam-welding device, because of the long regulator reaction time welding parameters, such as for example the contact resistance at the welding point (surface quality of the welding material), material properties of the welding material etc., cannot be sufficiently rapidly taken into account and there is also no possibility of adapting the welding current form to different welding conditions, e.g. to the requirements of the different materials being processed.
The hereinbefore mentioned EP-A2-0 260 963 proposes that a current source of high frequency be used, in order to make it possible to use a smaller welding transformer. As this causes problems with the necessary phase control of thyristors, a feed-forward or forward control of the welding current is operated, by using in a half-wave during the phase control a pre-calculated value which has been calculated beforehand on the basis of the measured value for the preceding half-wave. The arrangement which is known from this publication also does not operate satisfactorily in all working conditions as the welding current is likewise switched on and off only once per welding spot. As here also the regulator in each case operates with the measured value of the preceding welding spot, the reaction time of the regulator is relatively long. If the pulse length is varied during the pulse length modulation, the welding current form also varies, for which reason the latter cannot be adapted to a special material or to special operating conditions.
Common to both already known arrangements, moreover, is the fact that only the quadratic mean value of the welding current is measured as its actual value and therefore only the mean value of the welding current can be controlled. For this reason, a constant mean value of the welding current is preset as a nominal value.
From CH-A5-668 842 is known a device for the stepless control of the amplitude of a sinusoidal electric alternating current. In each half-wave of the alternating current, over a variable part thereof, controllable circuit elements can be moved from the blocking state into the transmitting state. Certainly, a type of electronically controllable adjustable transformer is provided thereby which is practically delay-free, but here also the possibility of influencing the welding current is limited to one switching procedure per half-wave thereof. For this reason, no more rapid regulating times can be obtained in this case also.
DE-C2-30 05 083 describes a process for the manufacture of longitudinal seam-welded rounded bodies in which, in order to obtain a continuous unbroken welding seam, the duration of one half-wave of the almost rectangular welding current is adapted to the time for conveying a body between the welding electrode rolls and the energy required during the welding operation can thereby be directly controlled by superimposing on the welding current a high-frequency current component. The possibility of regulation by superimposing a high-frequency current component is naturally limited, not only with regard to the regulating range but also the regulating time.
Finally, the specialist is familiar, for example, from the Soudronic publication "Electric Resistance-welding" MDI 00188 D, Pages 9 and 10, with varying the welding current strength by means of a phase-shifting control. Unfortunately, the welding current form also varies in each case. The same applies if the welding current is kept constant with varying load conditions, as in both cases the phase-shifting angel has to be varied. Moreover, the phase-shifting control of the primary alternating voltage of a welding transformer produces an interrupted welding current which is likewise disadvantageous.